1.5 Free-Fall and the Acceleration Due to Gravity

Free-fall means gravity alone acts

An object is in free-fall when the only force acting on it is gravity — no air resistance. Near Earth's surface, this produces a constant downward acceleration of g = 9.8 m/s², regardless of the object's mass. Heavier and lighter objects (ignoring air resistance) fall with the same acceleration.

Velocity changes, acceleration doesn't

During free-fall, velocity changes continuously (increasing downward or decreasing upward) while acceleration stays constant at g. At the peak of a toss upward, velocity is zero but acceleration is still -9.8 m/s² — acceleration never disappears just because velocity does momentarily.

Same kinematic equations apply

Free-fall motion uses the same kinematics equations as constant acceleration problems, just with a = -g (if up is positive). Objects thrown upward decelerate, stop, then accelerate downward, with symmetric time and speed going up vs. coming back down (ignoring air resistance).

Key terms

Free-fall

Motion where the only force acting on an object is gravity (no air resistance).

g (acceleration due to gravity)

Approximately 9.8 m/s², directed downward, near Earth's surface.

Acceleration at the peak of upward motion

Still equal to g (-9.8 m/s²) even though velocity is momentarily zero.

Does mass affect free-fall acceleration?

No — all objects fall with the same acceleration g, regardless of mass (ignoring air resistance).

Symmetry in free-fall (thrown up and back down)

Time and speed going up equal time and speed coming down to the same height, ignoring air resistance.

Try an AP-style question

A rock is dropped from rest. What is its speed after 2 s? (g ≈ 10 m/s²)

1. 10 m/s
2. 5 m/s
3. 40 m/s
4. 20 m/s